page_title: Discovery of the neutron
section_title: The nature of the neutron
hierarchical_section_title: Discovery of the neutron / The nature of the neutron
caption_attribution_description: English: A group of scientists participated in the 7th Solvay Conference, Bruxelles (October 1933). Seated (left to right): Erwin Schrödinger, Irène Joliot-Curie, Niels Henrik David Bohr, Abram Ioffe, Marie Curie, Paul Langevin, Owen Willans Richardson, Lord Ernest Rutherford (* 30. August 1871 in Spring Grove bei Nelson; † 19. Oktober 1937 in Cambridge, Vereinigtes Königreich), Théophile de Donder, Maurice de Broglie, Louis de Broglie, Lise Meitner, James Chadwick. Standing (left to right): Émile Henriot, Francis Perrin, Frédéric Joliot-Curie, Werner Heisenberg, Hendrik Anthony Kramers, Ernst Stahel, Enrico Fermi, Ernest Walton, Paul Dirac, Peter Debye, Nevill Francis Mott, Blas Cabrera y Felipe, George Gamow, Walther Bothe, Patrick Blackett, M.S. Rosenblum, Jacques Errera, Ed. Bauer, Wolfgang Pauli, Jules-Émile Verschaffelt, Max Cosyns, E. Herzen, John Douglas Cockcroft, Charles Drummond Ellis, Rudolf Peierls, Auguste Piccard, Ernest O. Lawrence, Léon Rosenfeld. Absents: Albert Einstein (emigr. USA) and Charles Eugène Guye
context_page_description: The discovery of the neutron and its properties was central to the extraordinary developments in atomic physics in the first half of the 20th century. Early in the century, Ernest Rutherford developed a crude model of the atom, based on the gold foil experiment of Hans Geiger and Ernest Marsden. In this model, atoms had their mass and positive electric charge concentrated in a very small nucleus. By 1920 chemical isotopes had been discovered, the atomic masses had been determined to be integer multiples of the mass of the hydrogen atom, and the atomic number had been identified as the charge on the nucleus. Throughout the 1920s, the nucleus was viewed as composed of combinations of protons and electrons, the two elementary particles known at the time, but that model presented several experimental and theoretical contradictions.
  The essential nature of the atomic nucleus was established with the discovery of the neutron by James Chadwick in 1932 and the determination that it was a new elementary particle, distinct from the proton.
context_section_description: The question of whether the neutron was a composite particle of a proton and an electron persisted for a few years after its discovery. In 1932 Harrie Massey explored a model for a composite neutron to account for its great penetrating power through matter and its electrical neutrality, for example. The issue was a legacy of the prevailing view from the 1920s that the only elementary particles were the proton and electron.
  The nature of the neutron was a primary topic of discussion at the 7th Solvay Conference held in October 1933, attended by Heisenberg, Niels Bohr, Lise Meitner, Ernest Lawrence, Fermi, Chadwick, and others. As posed by Chadwick in his Bakerian Lecture in 1933, the primary question was the mass of the neutron relative to the proton. If the neutron's mass was less than the combined masses of a proton and an electron (1.0078 u), then the neutron could be a proton-electron composite because of the mass defect from the nuclear binding energy. If greater than the combined masses, then the neutron was elementary like the proton. The question was challenging to answer because the electron's mass is only 0.05% of the proton's, hence exceptionally precise measurements were required.
  The difficulty of making the measurement is illustrated by the wide-ranging values for the mass of the neutron obtained from 1932–1934. The accepted value today is 1.00866 u. In Chadwick's 1932 paper reporting on the discovery, he estimated the mass of the neutron to be between 1.005 u and 1.008 u. By bombarding boron with alpha particles, Frédéric and Irène Joliot-Curie obtained a high value of 1.012 u, while Ernest Lawrence's team at the University of California measured the small value 1.0006 u using their new cyclotron.
  In 1935 Chadwick and his doctoral student Maurice Goldhaber resolved the issue by reporting the first accurate measurement of the mass of the neutron. They used the 2.6 MeV gamma rays of Thallium-208 (²⁰⁸Tl) (then known as thorium C") to photodisintegrate the deuteron
  In this reaction, the resulting proton and neutron have about equal kinetic energy, since their masses are about equal.  The kinetic energy of the resulting proton could be measured (0.24 MeV), and therefore the deuteron's binding energy could be determined (2.6 MeV − 2(0.24 MeV) = 2.1 MeV, or 0.0023 u). The neutron's mass could then be determined by the simple mass balance
  where md,p,n refer to the deuteron, proton, or neutron mass, and "b.e." is the binding energy. The masses of the deuteron and proton were known; Chadwick and Goldhaber used values 2.0142 u and 1.0081 u, respectively.  They found that the neutron's mass was slightly greater than the mass of the proton 1.0084 u or 1.0090 u, depending on the precise value used for the deuteron mass. The mass of the neutron was too large to be a proton-electron composite, and the neutron was therefore identified as an elementary particle.  Chadwick and Goldhaber predicted that a free neutron would be able to decay into a proton, electron, and neutrino (beta decay).
caption: Seventh Solvay Conference, 1933.
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